1. Field of the Invention
The present invention relates to communication data links of the type employed between airborne hub and ground and/or airborne spoke stations. More particularly, the present invention relates to a novel dual protocol system which optimizes data flow of all different types of messages without regard to their length.
2. Description of the Prior Art
It is known that airborne surveillance platforms are used to capture optical images, infrared images and/or radar images and to process the sensed information into a digital format for communication to ground stations and other airborne stations.
The most commonly used protocol for transmitting such data heretofore has been Time Division Multiple Access (TDMA). While there are variations of TDMA protocols, all allot a dedicated time slot to each of the spoke stations that is the only time in which the ground station may transmit requests, messages and data to the hub. The spoke stations are usually designed for always-on reception. It is recognized that TDMA works well when the data being transmitted to the hub is short enough to fit into the allotted time slot, but is bad for long messages that do not fit into the time slot and must be broken up and sent piecemeal over extended time periods in a plurality of the dedicated time slots. Further, no known TDMA protocol has the ability to be interrupted by a priority interrupt initiated at the spoke stations.
The protocol designated Demand Assigned Multiple Access (DAMA) has been used in data links. In this system, all spoke stations have no time slots, but must send a request to transmit to the hub station before access is granted. The preferred mode of operation allocates the channel to the spoke station that first requests access during a period when no other spoke station is transmitting to the hub. Usually, the allocation has no priority or time restraint and allows the transmitting spoke station to transmit its message in its entirety without regard to time. It is possible for the spoke station to determine from the data being transmitted from the hub station when the hub's receiver is available or the hub station can inform the spoke station when such access is available. While DAMA protocol solves the latency problem created by long messages in a TDMA environment, it is bad for short messages because the overhead for negotiating and allotting of the channel usually exceeds the time allotted for the TDMA time slots.
Further, it is possible that a remote spoke station cannot raise its request fast enough to compete with closely located spoke stations and is virtually excluded from achieving access to the hub station in a very busy environment.
It is known by those skilled in the data link communications art that TDMA provides the spoke stations with the ability to continuously resynchronize their timing for using spread spectrum signals and virtually eliminates acquisition time. In contrast thereto, DAMA protocol eliminates the ability for the system to rapidly resynchronize spread spectrum signals, thus, most DAMA protocol systems do not use the desirable spread spectrum signals to avoid the time penalty of having to acquire a spread spectrum signal that is out of synchronization and/or has a period of uncertainty longer than the message time.
It has been suggested that Code Division Multiple Access (CDMA) may be employed in data link communication systems. Such systems require the hub station to have a receiver for each spoke station. Alternatively, the hub station can receive and store all of the spoke station signals and subsequently demodulate and despread the signals using a demodulator/despreader whose speed is N times the plural receiver system speed, where N is the number of spoke stations. In a high-speed data link communication system, CDMA is not a viable solution.
Thus, it would be highly desirable to provide a protocol and system which eliminates the undesirable features of TDMA, DAMA and CDMA data link systems and optimize the throughput of data from spoke stations to a hub station in a high speed data link communication system.